1. Field of the Invention
The present invention relates to a multibeam antenna apparatus for use in radio astronomical fields, communications, and so on.
2. Description of the Related Art
A prior art multibeam antenna apparatus is disclosed in xe2x80x9cMultibeam antennaxe2x80x9d, Masaaki Sinji, Journal of IECE (The Institute of Electronics and Communication Engineers), 77, 5, pp. 544 to 551.
FIG. 7 is a block diagram showing the structure of a prior art multibeam antenna apparatus. In the figure, reference numeral 1 denotes a main reflector having a reflecting surface of rotationally symmetric shape, reference numeral 2 is a sub-reflector having a reflecting surface of rotationally symmetric shape, reference numeral 3 denotes a focused beam feeder, and reference numerals 3a to 3d denote focusing reflectors which constitute the focused beam feeder 3. Each of the two reflectors 3a and 3b has a mirror finished surface of rotationally quadratic surface, and each of the remaining focusing reflectors 3c and 3d has a mirror finished surface of planar shape. Furthermore, reference numeral 4a denotes a focal point of the focusing reflector 3a, reference numeral 4b denotes a focal point of the focusing reflector 3b, reference numeral 4c denotes an image focal point caused by the focusing reflector 3c, which corresponds to the focal point 4a, reference numeral 4d denotes an image focal point caused by the focusing reflector 3d, which corresponds to the focal point 4b, reference numeral 5 denotes a primary radiator array, reference numeral 5a denotes each of a plurality of primary radiators which constitute the primary radiator array 5, reference numeral 6 denotes a transceiver connected to the primary radiator array 5, reference numeral 7 denotes an elevation angle rotation axis, reference numeral 8 denotes a bearing angle rotation axis, and reference numeral 9 denotes an antenna pedestal for securing the focused beam feeder 3, the primary radiator array 5, and the transceiver 6.
Next, a description will be made as to the operation of the prior art multibeam antenna apparatus. The multibeam antenna apparatus as shown in FIG. 7 uses the primary radiator array 5, which consists of the plurality of primary radiators 5a, for the main reflector 1, the sub-reflector 2, and the focused beam feeder 3, which implement a single mirror finished surface structure, in order to measure electric waves from a plurality of celestial objects or satellites at the same time. Electric waves, which come from different directions and then reach the multibeam antenna apparatus at the same time, are reflected and focused by the main reflector 1, so that they reach the primary radiator array 5 by way of the sub-reflector 2 and the focused beam feeder 3, and are received by the plurality of primary radiators 5a corresponding to the respective directions in which the electric waves are travelling, respectively. Thus a multibeam is implemented. The plurality of primary radiators 5a are arranged so that the orientation of each of multiple beams which constitute the multibeam agrees with a desired direction in which a corresponding electric wave is travelling.
When celestial objects are observed from the ground by using the multibeam antenna apparatus, for example, the directions of the objects to be measured change during measurements because the positions of the celestial objects on the celestial sphere rotate around the North Pole or the South Pole of the heaven under the influence of the spin of the earth and so on. In this case, while changing the orientation of the main reflector 1 so that it agrees with the direction of the center of gravity of the plurality of objects to be measured, for example, and tracking these objects to be measured, the prior art multibeam antenna apparatus receives electric waves from the objects to be measured. Because a relation between the relative positions of the plurality of objects to be measured rotates around the North Pole or the South Pole of the heaven while being maintained on the celestial sphere, the direction of each of the plurality of objects to be measured when viewed from the antenna rotates with respect to the direction of the center of gravity of the plurality of objects to be measured, too. It is therefore necessary to relatively rotate the arrangement of each of the plurality of primary radiators 5a, which corresponds to an electric wave from each of the plurality of celestial objects, and it is necessary to rotate the whole of the primary radiator array 5 so as to make a view rotation correction.
Because the prior art multibeam antenna apparatus is constructed as above, an electric wave from each of a plurality of objects to be measured is focused, byway of the main reflector 1 and the sub-reflector 2, to a position in the vicinity of the focal point 4c, which corresponds to the direction in which the electric wave is travelling to the multibeam antenna apparatus. When each of the main reflector 1 and the sub-reflector 2 has a rotationally symmetric shape, if the directions in which electric waves from the plurality of objects to be measured are travelling to the multibeam antenna apparatus are rotationally symmetric with respect to the optical axis of the main reflector 1, the positions onto which the electric waves corresponding to the multiple beams are focused are also rotationally symmetric with respect to the optical axis of the main reflector 1. An electric wave travelling in each beam direction which has been focused in this vicinity of the focal point 4c continues to be travelling while spreading and is focused again in the vicinity of the focal point 4d after passing through the focused beam feeder 3.
The directions in which electric waves are travelling in the focused beam feeder 3, which correspond to the orientations of multiple beams, respectively, become rotationally asymmetric with respect of the optical axis of the focused beam feeder 3 because of the focusing reflectors of offset type. As a result, even if the positions onto which electric waves are focused before being incident upon the focused beam feeder 3 are rotationally symmetric with respect to the optical axis of the main reflector 1, the positions onto which the electric waves are focused after exiting from the focused beam feeder 3 do not become rotationally symmetric with respect to the optical axis of the focused beam feeder 3, but have a distorted pattern. A problem is therefore that even if the plurality of primary radiators 5a which constitute the primary radiator array 5 are arranged so that they are rotationally symmetric with respect to the optical axis of the focused beam feeder 3, the orientations of the multiple beams in the multibeam antenna apparatus do not become rotationally symmetric with the optical axis of the focused beam feeder 3 and there causes a distortion in the orientations of the multiple beams.
Another problem is that when rotating the whole of the primary radiator array 5 for view rotation correction, the orientation of each beam varies according to the rotation of the primary radiator array 5 because of the rotational asymmetry of the orientation of each beam.
The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a multibeam antenna apparatus capable of preventing an error from occurring in the orientation of each beam.
In accordance with an aspect of the present invention, there is provided a multibeam antenna apparatus including a primary radiator array having a plurality of primary radiators and a lens array having a plurality of wavefront transformation lenses corresponding to the plurality of primary radiators, respectively. Preferably, the lens array is placed in the vicinity of a front end of the primary radiator array. As an alternative, the lens array is placed in an electric wave propagation range of a focused beam feeder where multiple beams are spatially isolated from one another in terms of electric power.
Thus the multibeam antenna apparatus according to the present invention can prevent an error from occurring in the orientation of each of multiple beams which constitute a multibeam.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.